In combustion systems, such as gas turbines, furnaces, boilers and internal combustion engines, it is desirable to know if combustion is occurring. Because of the location or noisy environment that the combustion system may be operating in, it may be difficult to determine the presence or absence of combustion.
For example, in an oil or gas fueled turbine, fuel is fed into a combustion chamber within which an ignition flame is present. If the flame becomes extinguished, commonly referred to as a flame-out condition, it is a concern that fuel may continue to be fed into the hot combustion chamber without appropriate ignition. Damage to the turbine can occur if the fuel is then inappropriately ignited (e.g., ignition caused by something other than the ignition flame). Consequently, if the ignition flame is extinguished within the combustion chamber, it is important that the fuel feed into the combustion chamber is quickly terminated and, thus, limit non-combusted fuel build up.
A flame sensor is generally used for detecting the presence or absence of an ignition flame within a combustion chamber of a gas turbine. In a flame sensor, a photodiode or other sensing element can be positioned inside the flame sensor. Light from the flame can travel through a window and be focused by a lens onto the photodiode. The output of the photodiode is generally proportional to the intensity of the light that the photodiode detects.
Sensitivity of the flame sensor is often diminished by contamination on the outside of a sapphire window filtering or attenuating light from the flame. This attenuation can be indistinguishable from malfunction of the flame sensor itself or of a flame-out condition. Contamination such as ash, dust, sand, water, or large pieces of refractory can deposit on the window and block the light from entering the flame sensor.